Load-bearing structural members, such as metal aircraft skin, frame, and support members, have a tendency to crack at stress riser areas. Once formed, these cracks tend to propagate causing further damage. The end result could be failure of the members with consequent loss of equipment and productivity, and possibly even loss of life. In view of these serious considerations, very costly repair measures must be implemented to preserve the structural integrity of those members.
Several curative measures, if taken, would prove to be most advantageous in overcoming the problems associated with stress cracking in structural members: 1) to predict the formation of new cracks and the propagation of existing cracks at critical areas on the surface of a structural member; 2) to implement preventative measures to deter the formation and propagation of cracks in these areas; and 3) to detect the formation and propagation of cracks should the preventative measures fail.
Some of these curative measures have been implemented in the prior art. For example, a variety of approaches have been developed to retard crack growth in structural members. One approach, as disclosed in U.S. Pat. No. 5,023,987, involves using a repair patch to repair damaged areas on composite skin structures such as aircraft wings. The patch is unitary in form and is fabricated from the same composite material used to construct the aircraft wing. Once fixed to the aircraft wing, the patch reduces stress levels in the damaged area to retard further structural damage. Similarly, U.S. Pat. No. 5,034,254 discloses a patch for repairing damaged areas on load-bearing, composite panels. This patch conforms to the curvature of the panel on which it is to be fixed and restores the panel to nearly its original load-carrying capability.
One drawback associated with the use of repair patches is the cost and inconvenience associated with having to remove and then remount the patch when performing a visual inspection for crack formation and propagation.
U.S. Pat. No. 4,179,940 discloses a technique for predicting crack formation in patch-covered areas on a structural member without removing the patch. Under this technique, crack initiation prediction is determined as a function of strain measurements taken in a patch-covered area on the surface of a metal aircraft frame member as that member is stressed towards fatigue failure. The patch used to implement this technique contains two strain gauges which are in fluid communication with one another beneath a sealed, pressurized cavity. Unlike the repair patches discussed above, however, this patch is constructed from a material which has no load-bearing properties and thus is unable to reduce stress levels in damaged areas on the surface of a structural member to deter crack formation or propagation.
At best, the prior art offers only a partial solution to the problems associated with stress cracking in structural members. Some prior art references address the crack detection problem while others address the crack prevention problem. None, however, offer all of the curative measures discussed above. Accordingly, a need exists for a method and apparatus which provides a complete solution to the problems presented by stress cracking in structural members which is achieved by implementing a three-pronged approach --the prevention, detection, and prediction of the formation and propagation of stress cracks in structural members.